Field of the Invention
The present invention relates to a high zirconia electrically fused cast refractory which is excellent in heat cycle stability and bubble formability, less forms zircon crystals when used for glass melting furnaces, and can be used stably for a long time.
Statement of the Related Art
Electrically fused cast refractories (sometimes simply referred to also as refractories) have been used frequently as refractories for use in glass melting furnaces.
The electrically fused cast refractory is a refractory having high density and excellent corrosion resistance to molten glass, produced by melting a raw material formed by mixing main components such as alumina, silica, and zirconia and minor components such as sodium compounds and boric acid each by a predetermined amount in an electric furnace, casting the melts in a refractory mold, and cooling the cast product in an annealing material to solidify the same in the shape of the mold.
For example, a high zirconia electrically fused cast refractory containing 80% by weight or more of ZrO2 is used as the electrically fused cast refractory described above.
Since the high zirconia electrically fused cast refractory has a high ZrO2 content and a dense texture, the refractory has high corrosion resistance to all kinds of molten glass.
Further, since the high zirconia electrically fused cast refractory has a property of not forming a reaction layer at a boundary with the molten glass, it is excellent in that defects such as stones or cords are less formed in the molten glass.
Accordingly, the high zirconia electrically fused cast refractory is particularly suitable to production of high quality glass.
In the mineral structure of the high zirconia electrically fused cast refractory, a most portion thereof comprises monoclinic zirconia crystals in which a small amount of a glass phase fills the grain boundaries of the zirconia crystals.
However, the property of the high zirconia electrically fused cast refractory undergoes a significant effect depending on the kind and the amount of components that constitute the glass phase.
Generally, the glass phase of the high zirconia electrically fused cast refractory comprises oxides such as Al2O3, SiO2, Na2O, B2O3, and P2O5.
On the other hand, zirconia crystals of the refractory transform reversibly between a monoclinic system and a tetragonal system accompanying abrupt volumic change about at a temperature of 1000° C. (in the course of temperature lowering) to 1150° C. (in the course of temperature rising).
A high zirconia electrically fused cast refractory with no cracks (fracture) during production and in the course of temperature rising can be produced at a level of actual production by moderating a stress generated by the volumic change accompanying the transformation of the zirconia crystals by the flow of a glass phase that fills crystal grain boundaries.
In glass melting furnaces using the high zirconia electrically fused cast refractory, burners are often used as a heat source. In a burner combustion type melting furnace, burners are changed on every several ten minutes and the temperature at the surface of the electrically fused cast refractory rises and lowers on every change.
Accordingly, the high zirconia electrically fused cast refractory which is often used for several years undergoes a number of heat cycles.
When the high zirconia electrically fused cast refractory undergoes the heat cycles, silica (SiO2) as main components of the glass phase and zirconia (ZrO2) crystals sometimes react to form zircon (ZrO2.SiO2) crystals.
In this case, since the zircon crystals are formed in the glass phase, formation of the zircon crystals leads to relative decrease of the glass phase. Further, as the glass phase decreases due to growing or increase of the zircon crystals, abrupt volumic change of the zirconia crystals at a temperature of about 1000° C. to 1150° C. is less absorbed.
As a result, when the zircon crystals increase exceeding a certain level, a residual volume expansion of the refractory itself increases extremely to generate cracks due to deterioration of the strength of the refractory structure and sometimes finally result in pulverization.
Accordingly, a high zirconia electrically fused cast refractory that less forms zircon crystals and is stable against heat cycles has been demanded (first subject).
Further, even a high zirconia electrically fused cast refractory which less causes zircon crystals by heating or heat cycles to the refractory itself sometimes tends to form zircon crystals when the refractory is in contact with the molten glass.
Particularly, when the high zirconia electrically fused cast refractory is used for a melting furnace for non-alkali glass such as liquid crystal display (LCD) panel glass (which may be hereinafter referred to as liquid crystal glass), zircon crystals are often tended to be formed.
The zircon crystals are formed upon melting of the glass due to difference of the concentration of the constituent components between the molten glass and the glass phase of the high zirconia electrically fused cast refractory by substitution of the components with each other.
That is, components that suppress the formation of the zircon crystals in the high zirconia electrically fused cast refractory are diffused into the molten glass, or components tending to form the zircon crystals intrude from the molten glass into the refractory. It is considered that the formation of the zircon crystals in the high zirconia electrically fused cast refractory is promoted by the occurrence of one or both of the diffusion and the intrusion described above.
In a state where the zircon crystals are formed in the high zirconia electrically fused cast refractory used for the glass melting furnace and the amount of the glass phase is decreased, abrupt volumic change of the zirconia crystals at a temperature of about 1000° C. to 1150° C. becomes difficult to be absorbed.
As a result, when the refractory undergoes heat cycles due to heating during operation and change of operation temperature, the residual volume expansion of the refractory itself increases extremely, by which the strength of the refractory structure is lowered tending to cause cracks in the refractory. The refractory is eroded selectively from the cracked portion. When erosion proceeds further, pieces of the refractory intrude into the molten glass to sometimes deteriorate the quality of the glass.
On the other hand, when a high zirconia electrically fused cast refractory that less forms the zircon crystals even in contact with the molten glass is used as the furnace material, the zircon crystals are less formed and the refractory remains stable even undergoing heat cycles due to heating during operation or change of the operation temperature of the glass melting furnace and cracks are less formed. Further, in the course of temperature lowering when the production of the glass melting furnace is interrupted, further occurrence of cracks and growing of already formed cracks can be suppressed.
Accordingly, upon restarting of operation after interruption of the operation, the high zirconia electrically fused cast refractory can be used again without replacing the refractory.
As described above, it has been demanded for a high zirconia electrically fused cast refractory that less forms the zircon crystals even under the condition in contact with the molten glass (second subject).
Further, the high zirconia electrically fused cast refractory tends to form unsaturated oxides with the oxygen content being less than a theoretical value and forms a strongly reducing composition. Accordingly, oxides of metals such as Fe, Cu, and Cr contained as impurities in the starting material are tended to be reduced and present as metals. Accordingly, the high zirconia electrically fused cast refractory has a lower degree of oxidation and exhibits dark gray color. Then, under the condition in which the refractory is in contact with the molten glass, bubbles due to the reduced metals are tended to be formed (the state tending to generate bubbles is hereinafter referred to as bubble foamability is insufficient).
Particularly, in high quality glass such as liquid crystal glass, failure of products due to bubbles provided a subject in view of the quality. Accordingly, less bubble formable refractory under the condition in contact with the molten glass has been demanded (third subject).
The high zirconia electrically fused cast refractory less foaming zircon crystals and having low bubble formability has been investigated so far.
Japanese Unexamined Patent Publication (JP-A) No. H08(1995)-48573 proposes a high zirconia electrically fused cast refractory of high electric resistance and stable against heat cycles, comprising 85 to 96% by weight of ZrO2, 3 to 8% by weight of SiO2, 0.1 to 2% by weight of Al2O3, 0.05 to 3% by weight of B2O3, 0.05% by weight or more of Na2O, 0.05 to 0.6% by weight of Na2O and K2O and 0.05 to 3% by weight of BaO, SrO and MgO.
However, the refractory of JP-A No. H08(1995)-48573 contains much B2O3 and has insufficient bubble foamability. Further, the refractory does not contain CaO that controls the viscosity of the glass phase and stabilizes the glass phase and contains much MgO that remarkably promotes the formation of zircon crystals in the refractory itself and under the condition in contact with the molten glass.
Accordingly, while the refractory contains Na2O, K2O, BaO, and SrO that suppress formation of the zircon crystals in heating the refractory itself, since it contains MgO of remarkably promoting formation of the zircon crystals, suppression for the formation of the zircon crystals was insufficient. Further, under the condition in contact with the molten glass, Na2O and K2O tend to migrate into the molten glass and suppression for the formation of the zircon crystals was insufficient.
JP-A No. H09(1997)-2870 proposes a high zirconia electrically fused cast refractory with less cracks during production and due to heat cycles, comprising 89 to 96% by weight of ZrO2, 2.5 to 8.5% by weight of SiO2, 0.2 to 1.5% by weight of Al2O3, less than 0.5% by weight of P2O5, less than 1.2% by weight of B2O3, less than 0.3% by weight of CuO, more than 0.01 and less than 1.7% by weight of P2O5+B2O3, 0.05 to 1.0% by weight of Na2O+K2O, 0.01 to 0.5% by weight of BaO, less than 0.5% by weight of SnO2, and less than 0.3% by weight of Fe2O3+TiO2.
JP-A No. H09(1997)-2870 describes in the paragraph 0028 of the specification that “symbol “−” for the content represents the content of less than 0.01% by weight and this means no substantial content”.
That is, in JP-A No. H09(1997)-2870, the minimum unit of the content is 0.01% by weight.
Further, it is described in the paragraph 0019 that “the content of P2O5 and B2O3 in total is more than 0.01% by weight and less than 1.7% by weight. When the content of P2O5 and B2O3 is extremely small, such an effect cannot be provided”.
That is, in a case where the contents of P2O5 and B2O3 is extremely small, each of the contents is less than 0.01% by weight and the total contents is also less than 0.01% by weight.
On the other hand, in a case where the content of P2O5 and B2O3 in total is more than 0.01% by weight, P2O5 and B2O3 are contained each by 0.01% by weight or more and the total content of them is 0.02% by weight or more.
Accordingly, in JP-A No. H09(1997)-2870, the content of P2O5 is 0.01% by weight or more.
In JP-A No. H09(1997)-2870, addition of Na2O, K2O and BaO provides an effect of suppressing cracks during production and, even if P2O5 and B2O3 that promote the formation of the zircon crystals by heating are contained more or less, cracks are not generated and the formation, of the zircon crystals can be suppressed after the heat cycle test for the refractory.
However, while the refractory contains BaO that suppresses the formation of the zircon crystal, since it contains P2O5 that remarkably promotes the formation of the zircon crystals, suppression for the formation of the zircon crystals was insufficient in the heating of the refractory itself. Further, under the condition in contact with the molten glass, Na2O and K2O that suppress the formation of the zircon crystals tend to migrate to the molten glass and suppression of the formation of the zircon crystals was insufficient.
Further, when P2O5 is reduced, it tends to form iron-phosphorus compounds together with iron as the impurity in the refractory, which remarkably deteriorate the bubble foamability of the refractory when it is in contact with the molten glass.
Further, SnO2 is not an essential component and the effect of SnO2 against cracks during production and cracks after heat cycles is not described at all and the addition effect of SnO2 is unknown.
JP-A No. 2000-302560 proposes a less bubble foaming high zirconia electrically fused cast refractory of suppressing the formation of the zircon crystals comprising 86 to 96% by weight of ZrO2, 3 to 10% by weight of SiO2, 0.5 to 2% by weight of Al2O3, 0.05 to 3% by weight of Na2O, 0.05 to 0.3% by weight of B2O3 and 0.2% by weight or less of Fe2O3, CuO and Cr2O3 in total, and not containing P2O5.
In JP-A No. 2000-302560, bubble foamability is improved by restricting the content of Fe2O3, CuO, and Cr2O3 that cause bubble forming under the condition in contact with the molten glass.
However, since the B2O3 content was large, bubble foamability was insufficient. Further, under the condition in contact with the molten glass, Na2O tends to migrate to the molten glass and the suppression for the formation of the zircon crystals was insufficient.
JP-A No. 2007-176736 proposes a low bubble formability high zirconia electrically fused cast refractory of suppressing the formation of the zircon crystals comprising 87 to 94% by weight of ZrO2, 3.0 to 8.0% by weight of SiO2, 1.2 to 3.0% by weight of the Al2O3, more than 0.35% to 1.0% by weight of Na2O, more than 0.02% by weight and less than 0.05% by weight of B2O3, not substantially comprising P2O5 and CuO and in which the weight ratio of Al2O3 and Na2O is 2.5 to 5.0 thereby suppressing the formation of zircon crystals in the refractory itself.
In JP-A No. 2007-176736, the refractory is less bubble formable and can suppress the formation of the zircon crystals in the refractory itself. However, under the condition in contact with the molten glass, Na2O tends to migrate into the molten glass and it was insufficient to suppress the formation of the zircon crystals.
JP-A No. 2008-7358 proposes a high zirconia electrically fused cast refractory of high electric resistance excellent in heat cycle stability and comprising 87 to 96% by weight of the ZrO2, 0.1 to less than 0.8% by weight of Al2O3, 3 to 10% by weight of SiO2, less than 0.05% by weight of Na2O, 0.01 to 0.2% by weight of K2O, 0.1 to 1.0% by weight of B2O3, 0.1 to 0.5% by weight of BaO, less than 0.05% by weight of SrO, 0.01 to 0.15% by weight of CaO, 0.05 to 0.4% by weight of Y2O3, 0.1% by weight or less of MgO, 0.3% by weight or less of Fe2O3+TiO2, and less than 0.01% by weight of P2O5 and CuO.
In JP-A No. 2008-7358, since the content of Na2O is small and the content of B2O3 is large, bubble foamability was insufficient. Further, although BaO and SrO as components for suppressing the formation of the zircon crystals are contained, Na2O tends to migrate to the molten glass under the condition in contact with the molten glass and the effect of suppressing the formation of the zircon crystals was insufficient.
WO 2012/046785A1 discloses a high zirconia electrically fused cast refractory less foaming zircon crystals, comprising 86 to 96% by weight of ZrO2, 2.5 to 8.5% by weight of SiO2, 0.4 to 3% by weight of Al2O3, 0.4 to 1.8% by weight of K2O, 0.04% by weight or less of B2O3, 0.04% by weight or less of P2O5, and 3.8% by weight or less of Cs2O and not substantially comprising Na2O (0.04% by weight or less). However, since the contents of B2O3 and P2O5 for preventing cracks during production are extremely small and alkali metal oxides such as K2O and Cs2O of large ionic radius are contained by large amount, it was insufficient for stably producing large-sized products with less cracks during production and in the course of temperature rising. Further, since the Na2O content is small, bubble foamability was insufficient.
Further, Cs2O is extremely expensive to result in a problem in view of industrial productivity.
WO 2012/046786A1 proposes a high zirconia electrically fused cast refractory less precipitating zircon crystals, comprising 85 to 95% by weight of ZrO2, 2.5% by weight or more of SiO2, 0.04% by weight or less of Na2O, 0.04% by weight or less of B2O3, 0.04% by weight or less of P2O5, and SrO as an essential component, containing at least one of K2O and Cs2O, and satisfying the following equation (1) and equation (2):0.2≦0.638×C(K2O)+0.213×C(Cs2O)+0.580×C(SrO)/C(SiO2)≦0.40  Formula (1)0.10≦0.580×C(SrO)/C(SiO2)  Formula (2)in which C(X) represents the content (mass %) of the component X.
A high zirconia electrically fused cast refractory with no cracks during production and less forming zircon crystals even in contact with the molten glass by defining the molar concentration ratio of K2O, Cs2O, and SrO to SiO2 according to the formulae (1) and (2) was proposed.
However, since the contents of B2O3 and P2O5 that prevent cracks during production is very small and alkali metal oxides such as K2O and Cs2O having a large ionic radius are contained each in a large amount, it was insufficient for stably producing large-sized products with less cracks during production and in the course of temperature rising. Further, since the N2O content is small, the bubble foamability was insufficient.
Further, Cs2O is extremely expensive and results in a problem in view of industrial productivity.
As described above, high zirconia electrically fused cast refractory free of cracks during production of the refractory and in the course of temperature rising that can simultaneously provide effects of suppressing the formation of the zircon crystals even under a condition in contact with the molten glass, and low bubble formability cannot be obtained yet in prior art.